Anechoic chambers

An important application of absorbers is in the construction of anechoic chambers. Using today’s state-of-the-art absorbers and chamber designs, “free-space” with regard to amplitude and phase uniformity can be simulated to a very high degree. It is not uncommon at high microwave frequencies to be able to create a chamber test volume (quiet zone) in which the level of reflections from all regions is greater than 60 dB below the level of the incident signal. Chambers are customarily used to frequencies as low as 30 MHz and as high as 100 GHz. The lower frequency limit requires pyramidal absorbers, having thicknesses as great as 15 feet (4.6 meter). Most chambers are a mix of absorber thicknesses and types in various regions in order to achieve optimum performance and low cost. Chambers range in size up to 52 x 52 feet (15.8 x 15.8 meter) in cross section and to 175 feet (53.3 meter) in length. It is interesting that there are some existing anechoic chambers with lengths greater than 60 feet, which speaks well for their usefulness in terms of dollars invested. Chambers have been built for testing of objects as large as aircrafts and tanks.

While many chambers are in the shape of rectangular rooms, many others are in the shape of a horn and are known as tapered chambers. This shape effectively avoid specular reflections from the side walls, floor and ceiling which, in rectangular chambers, represent the primary limitation to effective performance at lower frequencies. Many chambers are built in shielded enclosures, where isolations up to 120 dB are effective in preventing the radiation of high power into or from the chamber.

The most significant advance with regard to chambers in recent years relates to the flammability of the microwave absorbers. As the result of several fires, the U.S. Naval Research Laboratory developed a set of standards for absorber quality, which greatly decreased the possibility of fire. These tests establish limits with regard to case of ignition and volume of noxious gasses given off during combustion. More specifically, they establish that chamber absorbers:

Will not ignite when probed with 220 volts AC.

Will not ignite when exposed to 2,000°C flame for 30 seconds.

Will not ignite when a cartridge heater at 600°C is inserted into the material for 10 minutes.

That concentrations of CO, HCL and HCN not exceed certain limits for specified test conditions.

Chambers are employed primarily for the following measurement purposes:

Both primary and secondary antenna patterns.

Both mono-static and bi-static radar cross section patterns.

Radome bore-sight error.

Complete systems, such as satellites, radars, aircrafts, computers, missiles, vehicles and electronic devices are measured in chambers with regard to such characteristics as comparability, susceptibility, vulnerability, system sensitivity, effective radiated power, tracking ability and bore-sight accuracy. Chambers provide a standard, reproducible environment for the measurement of a wide variety of electrical and electronic devices to establish that they meet requirements concerning spurious, harmonic and noise emissions. A sampling of such devices that are measured in chambers are microwave-ovens, communication equipment, typewriters, motor generators, lights, computers, relays, television sets etc.

Antenna pattern

Today, in the face of rapidly increasing electromagnetic pollution and proliferation, it becomes increasingly important that both transmitting and receiving antennas have low side lobe levels and narrow beam widths in order to reduce interference. Absorbers make a valuable contribution to this respect to a class of antennas known as tunnel antennas. These are antennas constructed with a forward extending lip or tunnel, which is covered with absorbers on the inner surface. Where such tunnels have a length in the order of twice the antenna aperture size, appreciable narrowing of the main lobe can be achieved with only small gain reductions. Such a tunnel can reduce side lobe drastically. Even relatively short extensions of only a few wavelengths in extent can provide side lobe reductions. While this echnique has been used most commonly with parabolic reflector antennas, a variety of antenna types, including planar arrays and horns, have been so adapted in the case of horns, the absorber generally is continued forward at the flare angle of the horn rather than parallel to the direction of propagation. For this purpose, the absorbers have also been mounted directly on the inner surface of the horn.

Tunnel antennas may be thought of as operating to provide a smooth amplitude taper transition across the aperture to zero at the edge. This is conducive to reduction of diffraction around the edge into the side lobe region.

Absorbers are useful in a number of other areas with regard to antennas. They are used to cover feeds, feed-mounting struts, transmission lines and the rear surfaces of antennas. They are customarily used in cavity-backed spiral antennas to prevent radiation of a circularly-polarized signal of the opposite sense. They are used to cover surfaces or objects adjacent to antennas to prevent antenna pattern degradation.

Radar cross section

Probably the first application of absorbers was during World War II for the purpose of achieving radar camouflage of submarine snorkel and periscope. In the early 1940’s, effective signal reductions with both resonant and broadband types were demonstrated.
Since that time, the information on such applications has disappeared in view of the significance to the military. There have been references, however, in the open literature to application of absorbers to parts of aircraft such as jet engine inlets.